Currently the three overcommit ratios: ram_allocation_ratio,
cpu_allocation_ratio, and disk_allocation_ratio can be set to negative values.
It's up to the scheduler filters to impose the checks on these ratios.
It makes more sense to make sure these 3 ratios are not negative when
nova-compute is started. If any of these ratios is negative then nova-compute
service will fail to start.
Closes-Bug: #1604116
Change-Id: Ic960e319b59910c0178e81259b2e35435f2db592